Ravenloft - Curse of Strahd

Sleep

The Realm of Dread causes even the hardy to sleep restlessly and have nightmares. People constantly have a feeling of being watched when not indoors and behind the safety of a locked door. It is not uncommon for characters to spend a goodly portion of their life in a more or less sleep-deprived state. After some particularly frightening or harrowing encounters, the DM may require you to make a Constitution (fortitude) or Charisma (willpower) save before resting (long or short).

If you fail the save for a short rest, you fail to calm your mind and are distracted during your rest. Any hit dice used to restore lost hit points are less effective (roll all hit dice, divide total in half, rounding down).

If you fail the save for a long rest, your sleep is not beneficial due to tossing, turning and nightmares. You regain all hit points as normal, but you also gain one level of exhaustion.